1. Field of the Invention
The present invention relates to high accuracy diffraction interferometry, and more specifically, it relates to the use of embodiments of the Phase Shifting Diffraction Interferometer to measure the absolute aberrations of optical flats.
2. Description of Related Art
An optical flat is generally a piece of glass (e.g., fused silica, BK7, Zerodur, ULE), having one or both surfaces carefully ground and polished piano, generally flat to less than a fraction of a wavelength. Optical flats play two important roles: as optical elements and as optical reference standards. Most optical systems contain flat-surfaced optical elements (mirrors, windows, plano-lenses, gratings, laser amplifier disks, etc.). Optical flats are also the basic standards that are used as references in interferometers to measure and qualify other optics. The absolute flatness of optical flats therefore becomes the limit on how well other optics can be fabricated.
Measuring the flatness of an optical flat at first may appear trivial—but a reference surface is needed to which the optical flat can be compared. An example is a Fizeau interferometer, shown in FIG. 1 (Prior art). A laser 10 provides a beam 12 that passes through a (second surface) beamsplitter 14. The beam is expanded and recollimated by telescope 16, which comprises a first positive lens 18 (for expanding the beam) and a second positive lens 20 (for collimating the beam). The enlarged beam 22 then passes through a reference flat 24 and is reflected from an optical flat 26 under test. The beam is reflected from the optical flat 26, then travels back towards the beamsplitter 14, from which it is reflected and imaged by an imaging lens 28 onto a charge coupled device (CCD) 30. The reference flat (auxiliary optic) 24 is used as a standard against which the optical flat 26 is compared. The reference flat contains unknown errors that limit the accuracy of measuring the true flatness of the optical flat Trying to independently measure the reference flat presents the same limitations as measuring the optical flat 26 itself. These measurement errors can be reduced (but never be completely eliminated) by averaging a number of measurements with the reference flat 24 at different lateral positions.
The surface of a liquid has been used as a reference flat, but problems with vibrations, thermal gradients, inhomogenities, surface particles (dust), the meniscus due to the walls of the container, and the curvature of the earth are all problems that limit the accuracy of such a measurement.
Over the years many techniques have been proposed to determine the absolute surface figure of optical flats using inter-comparisons between several optical flats. The common interferometric “three flat test” has been in use for decades, but this test only gives an absolute point-by-point measurement along one diameter of a flat—not over the surface area. Many researchers have tried to extend this technique by taking additional data with the flats at several rotational positions. This technique requires a polynomial fit (an approximate representation of the surface) that covers an area; however, this method does not provide accurate point-by-point information over the area.
The absolute measurement of the surface figure of an optical flat has been the subject of study ever since the invention of the interferometer over 100 years ago. Until now there has been no way found to make an absolute point-by-point measurement over an area on an optical flat Therefore, a technique is desirable that gives point-by-point measurements over such an area.